1. Field of the Invention
This invention relates to a radio frequency (RF) signal amplification system producing an amplified signal with reduced distortion.
2. Description of Related Art
The relation between the output and the input of a RF power amplifier may be linear or non-linear depending on the amplitude of its input signal. xe2x80x9cLinearxe2x80x9d refers to the amplifier gain and phase shift being constant. xe2x80x9cNon-linearxe2x80x9d means both gain and phase shift are not constant. A distorted spectrum will be generated at the output of an amplifier if it operates in non-linear region. For example, if an input signal is a two single frequency carrier, an amplifier operating non-linearly produces multiple intermodulation (IMD) components at its output. If the input signal is a modulated channel with a definite bandwidth, the spectrum shape will be distorted (expanded, or what can be referred to as spectral regrowth) at the output.
Here, amplifier non-linearity is viewed from a spectral point of view. As such, no matter the type of input signal to be amplified, whether single frequency carriers or modulated channels, amplifier distortion will be described by spectral regrowth (including intermodulation components or distorted spectrum shape). The amplifier output waveform (spectrum) will be divided into xe2x80x9cdistortion spectrumxe2x80x9d and xe2x80x9cnon-distortionxe2x80x9d spectrum. The latter is the part with the same shape as the input signal and the former is the regrown spectrum.
Spectral regrowth directly raises the ACPR (adjacent channel power suppression ratio) and raises interference to adjacent channels. In most of wireless communication systems there are strong limitations to ACPR, and therefore, RF amplifiers have to be linearized (to reduce spectral regrowth) to meet communication requirements.
The simplest way to keep a radio frequency (RF) amplifier working linearly is to keep its output power much lower than its P1dB (1 dB output power suppression point) level. Unfortunately, in this case, the amplifier efficiency is very low. For example, a base station system requires ACPR of xe2x88x9256 dBc. If an RF power amplifier has P1 dB=40 dBm and IP3 (third order intercept point) of xe2x88x9250 dBm, this amplifier can only operate to output 22 dBm to satisfy the ACPR requirement. This output level is 18 dB lower than its rating level. In this case, the amplifier efficiency will be very low (5% or less).
Various linearization methods are used to enable the use of more cost-effective and more power efficient amplifiers while maintaining an acceptable level of linearity. For example, predistortion techniques are commonly used to improve the performance of RF power amplifiers. Predistortion techniques distort the input signal prior to amplification by taking into account the transfer function characteristics for the amplifier. Digital predistortion techniques can linearize RF amplifiers effectively, but the circuit is complicated and costly. Furthermore, predistortion systems have to cover a wide bandwidth which is much wider than the bandwidth of the input signal, and therefore, there is difficulty in getting fast digital processing processors for real time linearization in case of wideband applications.
Feed-forward correction is another approach for linearization of RF power amplifiers. The basics of this technique is to cancel distortion on the output side of the amplifier. There are normally two additional loops in such kind of circuits. One loop is to obtain the amplifier distortion and the other loop is to amplify the distortion signal. The amplified distortion signal is fed forward to the output side of the amplifier and is used to cancel the distortion in the delayed main amplified signal. Because the cancellation is taken at the output side of the amplifier, and the fed forward distortion signal is coupled to the main signal path through a coupler, the power needed for the feed forward distortion signal is quite large. As such, an additional power amplifier for the distortion signal is needed. The need for the high power distortion amplifier (error amplifier) will considerably reduce the total efficiency of the amplifier module, and the error amplifier itself will introduce additional non-linearity problems.
Thus, other improved amplifier linearization techniques are desired, especially one which can reduce distortion over a wide frequency band of operation and maintain a reasonably high efficiency.
The present invention is an amplifier distortion reduction system which obtains a distortion signal from the amplifier output and feeds the distortion signal back to the input side of the amplifier to cancel with the distortion produced at the amplifier output. For example, a signal to be amplified by an amplifier is received on a main signal path. The amplifier produces an amplified output with a non-distortion spectrum and a distortion spectrum. A sample of the amplified output is produced from the main signal path and placed on a feedback path. On the feedback path, the distortion spectrum is obtained from the sample amplified output. The distortion spectrum is phase and/or amplitude adjusted to produce the distortion signal. The distortion signal is placed onto the main signal path at the input side of the amplifier with the signal to be amplified to destructively combine with the distortion produced from the amplifier in amplifying the signal to be amplified.